Many attempts have been made in steel plants to achieve a continuous production line throughout the various stages of steel production, not only for the purpose of increasing product yields and production efficiency but also for the purpose of saving energy. Of these, the most significant is an attempt to create a continuous hot rolling line in which hot rolled steel strip (hot coil) is produced.
Heated slabs are reduced one by one intermittently by passing through a rough rolling mill and a finish rolling mill according to the prior method of hot rolling.
An intermittent rolling method like this easily gives rise to distortion of the leading end of the steel stock particularly when it is thin and stacked up upon the guide rolls, or is wrinkled at the rear end of the steel stock and damages the roll as it leaves the mill.
The rolling line will have to be stalled for maintenance and/or repair or exchange of rolls, if problems of this kind occur.
One way of avoiding the above mentioned difficulty is to slow down the rolling speed at the leading end of the hot steel stock. However, this has to be carried out at the sacrifice of the mill efficiency and, in addition, it can easily give rise to discrepancies in product quality and consequent defects as the rolling temperature is taken out of the adequate range for rolling.
Furthermore, a failure to meet the dimensional tolerances is often caused at the leading and rear ends of the steel plate by unstable tension created at the ends. These portions have to be discarded, resulting in an appreciable loss of the product yield.
A number of methods, as mentioned below, have recently been proposed in order to solve these problems.
Basically, the rear end of a preceding rough rolled steel plate (hereafter referred to as the preceding plate) and the front end of the succeeding rough rolled plate (hereafter referred to as the succeeding plate) are joined together to provide a continuous supply of steel stock to the finishing mill.
Japanese Patent Kokai No. 61-1489 discloses the most basic procedures for lap welding the ends of descaled steel sheets (sheet bar). Japanese Patent Kokai 61-126938 and Kokai 61-126907 also propose a method for pressure welding after descaling with the aid of hot scurfers. Sufficient joint strength is secured by neither of these methods, because oxidation takes place again immediately after descaling, forming oxide scale on the joining surfaces due to the high temperature of the steel plates themselves before pressure welding is completed.
The technology disclosed by Japanese Patent Kokai No. 60-244406 shows a method of preventing re-oxidization of the surfaces to be joined after descaling, by blowing them with inert gas (such as nitrogen or argon). Even this method cannot always be perfect, as surface re-oxidation due to air binding from the surrounding atmosphere is inevitable, and together with decreasing the temperature of the surfaces to be joined by gas blow may prevent sufficient development of the joint strength.
The inventors of this invention discovered that a separate step of mechanical descaling prior to the out joining of hot steel stocks is not necessary if the joining process is carried out in a specific reducing atmosphere. The invention based on the above findings has been disclosed in Japanese Patent Kokai 5-8058 (U.S. Pat. No. 5,222,652).
The method according to the invention of Japanese Patent Kokai 5-8058 (U.S. Pat. No. 5,222,652) is characterized by pressure welding of the hot steel stock after heating the joining portion of the stock in a reducing atmosphere and after the oxide scale is reduced.
However, there is a limiting thickness of the scale which can be reduced simply by heating in a reducing atmosphere, and the reduction of a scale heavier than 10 .mu.m or a scale of the composition that is not easily reduced will be take long time.
Joining of the hot steel stock in the hot rolling line should be carried out in as short a time as possible. Since the hot steel stock after hot pressure joining is exposed to tensile stress in the continuous finish rolling, the joint ought to have an elevated temperature tensile strength to withstand this tension.
In some cases, however, the joint may fall short of the required strength with a simple reduction of the scale in a reducing atmosphere.